1. Field of The Invention
The present invention relates to a torque converter with a lock-up mechanism for automatic transmissions, which includes two torsional dampers capable of effectively damping or absorbing torsional vibration transmitted from an engine crankshaft through a converter cover to a transmission input shaft, due to fluctuations in torque transmitted from the crankshaft to the torque converter.
2. Description of The Prior Disclosure
As is generally known, the conventional torque converter for automatic automotive transmissions comprises a converter cover, a pump impeller integrally connected to the converter cover, a stator, a turbine runner firmly connected through a turbine hub to a transmission input shaft, and a substantially disc-like drive plate interconnecting the crankshaft and the converter cover by means of a plurality of fasteners, such as bolts so that torque generated by an internal combustion engine is directly transmitted from the crankshaft acting as an output shaft to the pump impeller through the converter cover.
Recently, there have been proposed and developed various torque converters which include a lock-up mechanism which is operable for directly and mechanically interconnecting the turbine runner and the converter cover to transmit torque from the crankshaft directly to the turbine runner during vehicle operation at speeds greater than a predetermined vehicle speed.
For example, a torque converter with a lock-up mechanism having only one torsional damper disposed between the drive plate and the converter cover, has been disclosed in Japanese Utility Model (Jikkai) Showa 58-79156.
Furthermore, another torque converter with a lock-up mechanism having a lock-up torsional damper disposed in the converter cover, has been disclosed in U.S. Pat. No. 4,305,487 entitled "LOCK-UP TORQUE CONVERTER WITH DAMPER" which was granted on Dec. 15, 1981 and assigned to "NISSAN MOTOR COMPANY, LIMITED" this disclosure corresponds to Japanese First Publication (Tokkai) Showa No. 54-132060. In general, the lock-up mechanism for a torque converter of this type includes a lock-up clutch piston slidably fitted on the outer periphery of the turbine hub, an annular clutch facing attached to the perimeter of the clutch piston to establish or terminate the engagement between the inner peripheral wall of the converter cover and the mating surface of the clutch facing, a torsional damper being comprised of a plurality of torsion springs and operably connected between the clutch piston and the turbine hub to provide a driving connection, and a lock-up control valve provided for controlling the operation of the clutch piston. When the vehicle is running at speeds greater than a predetermined vehicle speed, the lock-up mechanism operates in such a manner that the clutch facing on the clutch piston engages the inner peripheral wall of the converter cover. As a result, torque generated by the internal combustion engine is transmitted from the crankshaft through converter cover to the clutch piston, and then from the clutch piston through the torsional damper via the turbine hub to the input shaft. In this manner, during activation of the lock-up mechanism, the output shaft of the torque converter is directly and mechanically connected to the transmission input shaft thereof thereby enabling the vehicle to reduce fuel consumption. However, in the above mentioned torque converters with lock-up mechanisms, torsional vibrations due to fluctuations in torque transmitted from the crankshaft to the drive plate are not sufficiently absorbed, particularly, at low vehicle speeds. For instance, at low revolutions, the engine causes torque fluctuations resulting in a relatively high level of torsional vibration intensity in a drive system, the drive system being comprised of the crankshaft, the torque converter and the input shaft. Under these conditions, the torsional vibration is in part absorbed by the torsional springs operable between the clutch piston and the turbine hub, but this is insufficient. Therefore, the spring constant of the torsional springs, or the torsional rigidity of the torsional damper may traditionally be varied to control the resonance frequency of the above mentioned drive system.
FIG. 5 is a graph representative of the relationship between torsional vibration intensity and the frequency of a torsional vibration at two different torsional rigidities with regard to a conventional torque converter having a lock-up mechanism. In FIG. 5, the curve A designates torsional vibration characteristics of the drive system at a torsional rigidity of K.sub.1 =6 kgfm/deg, where K.sub.1 is representative of the torsional rigidity of the torsional damper used in the lock-up mechanism. Curve B designates other torsional vibration characteristics of the drive system at a torsional rigidity of K.sub.1 =1 kgfm/deg. As clearly seen in FIG. 5, the resonance frequency of the drive system is slightly lowered from 58 Hz to 41 Hz, in accordance with the change in the torsional rigidity from 6 kgfm/deg to 1 kgfm/deg. However sufficient reduction of the torsional vibration intensity cannot be obtained only by decreasing the torsional rigidity of the torsional damper applied in the lock-up mechanism. As appreciated from FIG. 5, the torsional vibration intensity is high at low revolutions of the engine because fluctuations in torque generated by the engine are greater at lower engine revolutions. As a result, if the lock-up mechanism is operated at low revolutions, a high torsional vibration intensity is transmitted from the crankshaft to the input shaft, in addition to the torque from the engine thereby causing high level transmission whine or operating noise which results in discomfort for the vehicle occupants. For this reason, as is generally known, the lock-up mechanism is designed so as to operate only at relatively high engine revolutions where torque fluctuations are minimal, that is, the rotation speed of the engine is quite steady.
In this manner, since the lock-up control valve engages the clutch piston with the converter cover within the above described high revolution range, the lock-up mechanism can prevent torque loss occurring between the pump impeller and the turbine runner only within this high revolution range. In other words, the lock-up mechanism of a conventional torque converter can operate only within a relatively narrow revolution range. To accomplish lower fuel consumption than conventional torque converters with a lock-up mechanism, it is desirable that a lock-up mechanism operate within a wider range of revolutions without causing discomfort to the vehicle occupants due to increased operating noise caused by high levels of torsional vibration.